Golden paintbrush (Castilleja levisecta) COSEWIC assessment and status report: chapter 6

Biology

General

There is extensive published information on the reproduction and dispersal, germination, seedling ecology, survivorship, herbivory and physiology of golden paintbrush in Canada and adjacent areas of the United States. Much of the information from U.S. studies is summarized by Caplow (2004) and Chappell and Caplow (2004). Information on the phenology and population biology of Canadian populations is found in Fairbarns (2005a).The average age of mature individuals in the population has not been determined.

Life cycle and reproduction

Shoot dormancy begins to break as early as mid-September, when the soil remoistens following the end of the summer drought. By January, most shoots have broken dormancy. During the winter months, most fresh shoots merely consist of a congested mass of reduced leaves at the base of the plant. Most shoots begin to elongate in March, as the leaves fill out and internodes elongate. The first floral buds can be detected as early as March and flowering peaks in April and May, although flowering usually continues into June and flowers may continue to develop well into July in favourable years. Green fruits develop from May to July and ripe, undehisced fruit are usually present from July to early September. Most capsules begin to dehisce from late August through September and seeds are gradually shed until late November or December, at which point most ripe seeds have dispersed. The seeds are minute, lack adaptations to assist in long-distance dispersal, and appear to be shed when wind shakes the ripened plants. Canadian plants produce an average of 35 capsules per genet and capsules had an average of 80 seeds (Fairbarns 2005a). Seed viability varies among populations and germination rates of 39%-96% were observed in fresh seeds of plants from south Puget Sound (Wentworth 1994, Caplow 2004, Kaye 2001).

Seedlings are inconspicuous in the field and germinants are rarely observed in natural conditions (pers. obs., Pearson pers. comm. 2005). Experimental seeding with up to 1,000 seeds per square metre have resulted in very low rates of germination and establishment (Fairbarns 2005b, Pearson and Dunwiddie 2003).

Golden paintbrush, as a taprooted perennial, is incapable of clonal growth by asexual reproduction.

Herbivory

Tent caterpillars (Malacosoma sp.) and spittlebugs (Cercopidae) have been observed feeding on Canadian plants. The former tend to cause moderate damage to a small proportion of the population while the latter are not abundant and likely have little effect (pers. obs.). Mammals, including deer, rabbits and voles are also reported to graze on golden paintbrush in some areas (USFWS 2000) and may have played a role in the loss of some Canadian populations, but the extant Canadian populations occur on islands which lack mammalian herbivores.

Interspecific interactions

Members of the genus Castilleja are hemiparasites, containing chlorophyll and fixing carbon through photosynthesis but receiving water and nutrients through parasitic root connections (Kuijt 1969). Despite their photosynthetic abilities, some related hemiparasites may also obtain photosynthates from their hosts (Atsatt 1970). Many hemiparasites may obtain secondary compounds from their host plants. These can reduce herbivory without affecting pollinators. It appears that alkaloids may be taken up in the leaves and outer floral tissues of some species but not by their nectar, which has the ultimate effect of increasing seed production and improving fitness (Adler 2000, Adler and Wink 2001, Boros et al. 1991).

Related hemiparasites in the Castillineae have been shown to be facultative hemiparasites capable of growing and producing flowers in the absence of a host, but shoot mass tends to be much higher in the presence of a host (Matthies 1997).

Grassland relatives in the genus Orthocarpus form haustorial connections (root grafts) with a number of species including annuals and perennials of the legume, grass, composite and many other families. Not all plants are equally good hosts ─ some appear to reduce reproductive potential. The parasitic habit is unlikely to be a limiting factor given the diversity of potential hosts intermixed within the Canadian populations of golden paintbrush.

Species of Castilleja may serve as a secondary host for larvae of the Taylor’s Checkerspot (Euphydras editha taylori) if its primary food source (Plantago spp.) withers before the onset of diapause (Vaughan and Black 2002). This butterfly (Endangered in Canada) formerly occurred on Trial Island, where golden paintbrush may have been an important food source.

Fruit set is low in the absence of pollinator visitation (Wentworth 1994) but pollinators (primarily bees) do not appear to be limiting in extant Canadian populations (pers. obs.).

Adaptability

Plants have been successfully propagated from seed and outplanted in the autumn (Caplow 2004).

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